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The instrument almost universally used in determining 

 the time is the Astronomical Transit Instrument. Through 



o 



the courtesy of the Director of Harvard College Obser- 

 vatory, we have before us to-night, a very beautiful speci- 

 men of this instrument, built by M. Herbst of Poulkova. 

 You notice that it has but one motion, simply around this 

 axis which points east and west, and makes a right angle 

 with the telescope tube. Now as I take hold of the tele- 

 scope, you see the telescope only moves from the north 

 to the south, that is, in the meridian. If we suppose this 

 axis to be perfectly horizontal, and this delicate level 

 which hangs from it will tell us if it is not so, I think 

 you will readily see that the astronomer has only to point 

 the instrument so that it will have the same altitude as a 

 star approaching the meridian, in order to have that star 

 visible in the telescope as it crosses it. Now if we imag- 

 ine the star to be exactly in the centre of the field of 

 view of the telescope to-night, and if we do not move the 

 telescope, to-morrow night at about this time, the same 

 star will re-appear, and the interval between its two suc- 

 cessive appearances is one sidereal day. 



Unfortunately for our purpose Nature has not provided 

 a visible meridian line, and accordingly the astronomer 

 contrives a visible substitute. We have on the screen a 

 circle of light, which fairly represents the circle of light 

 you would see by placing your eye to the telescope. You 

 will understand that it will take several minutes for the 

 star to cross this circle of light, or what is the same thing, 

 the field of view of the telescope. To tell when the star 

 is in the middle of this field there is inserted in the tele- 

 scope a fine spider's web, which divides the field of view 

 into two parts, and which represents to the astronomer, 

 as nearly as practicable, the meridian line on the heavens. 



Practically instead of one line there are a number of 



